1. Field of the Invention
The present invention relates to an electromagnetic focus circuit, used for a cathode-ray tube, which generates electric current waveform of electric current to be supplied to a focus coil by ON/OFF-controlling of a plurality of switches.
2. Description of the Related Art
FIG. 1 is a configurational view of a conventional electromagnetic focus circuit. The electromagnetic focus circuit 100 connects the first and second voltage-variable power sources 101, 102 and a focus coil 103, through which electron beams forming raster pass, to each other. The electromagnetic focus circuit 100 generates electric current waveform synchronized with horizontal synchronization signals by ON/OFF-controlling of four switches 104 to 107.
A microcomputer 108 establishes predetermined ON/OFF-timings of the switches 104 to 107 on the basis of a synchronization pulse when detecting the synchronization pulse of the horizontal synchronization signal S1. Pulses are issued to the corresponding switches 104 to 107 from four pulse generation circuits 110 to 113 in compliance with the established timings. The respective switches 104 to 107 are controlled in response to the pulses supplied thereto.
As the switches 105, 106 are simultaneously turned on, the second voltage-variable power source 102 and focus coil 103 are connected to each other. Electric current which linearly increases in proportion to time is caused to flow from the second voltage-variable power source 102 to the focus coil 103. Furthermore, as the switches 104, 106 are simultaneously turned on, the first and second voltage-variable power sources 101, 102 and focus coil 103 are connected to each other. Electric current which linearly increases in proportion to time is caused to flow from the first and second voltage-variable power sources 101, 102 connected in series to each other to the focus coil 103. At this time, the ratio of an increase of electric current is proportionate to the voltage of the power sources.
A counter electromotive force is produced in the focus coil 103 when the switch 104 is turned off, and a diode 115 is turned on by the counter electromotive force. The focus coil 103 is short-circuited by the diode 15 and switch 106, whereby a constant current is caused to continuously flow into the focus coil 103. Subsequently, if the switch 106 is turned off, a counter electromotive force generated at the focus coil 103 turns on a diode 116. Electric current is caused to reversely flow from the first and second voltage-variable power sources 101, 102 connected in series to each other, to the focus coil 103. The electric current is decreased in inverse proportion to time. Continuously, as the switch 107 is turned on, the second voltage-variable power source 102 and focus coil 103 are inversely connected to each other, wherein the electric current flowing in the focus coil 103 is decreased in inverse proportion to time. At this time, the ratio of a decrease of the electric current is proportionate to the power source voltage. At the moment when no electric current flows into the focus coil 103, the counter electromotive force of the focus coil 103 becomes zero. As a result, the diode 115 is turned off. Since the power supplied from the first and second voltage-variable power sources 101, 102 is regenerated, the mean value of the power supplied from two voltage-variable power sources 101, 102 becomes very small.
While horizontal scanning is carried out by an electron beam, it is preferable in order to continuously cause the focus of the electron beam to the fluorescent surface that the ideal electric current waveform is supplied to the focus coil. The conventional electromagnetic focus circuit supplies to the focus coil an electric current waveform which is as proximate to the ideal electric current waveform as possible, through an ON/OFF-controlling of the switches.
However, since the abovementioned conventional electromagnetic focus circuit has no function to limit the electric current in the circuit, there is such a shortcoming that overcurrent flows into the focus coil 103 if the voltage of the first and second voltage-variable power sources 101, 102 is too high with respect to the cycle of the horizontal synchronization signal S1.
Furthermore, if the switches 105 and 106 or the switches 106 and 107 are simultaneously turned on, there is such a problem that the voltage-variable power sources 101, 102 are short-circuited.
Furthermore, in a case where an effective picture term in the horizontal direction changes, strains may be produced in the approximate current waveform, and the amplitude of the electric current waveform may change. Furthermore, in a case of diagonal projection, the focus current accuracy may change below and above the picture screen. Resultantly, such a problem arises, by which the difference between the ideal current waveform and the actual waveform becomes large, resulting in a lowering of the picture quality.